Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and lost cost time domain reflectometry (TDR) instruments.
In a TDR instrument, a very fast (about 1 nanosecond) electric pulse with a rise time of 500 picoseconds, or less, is propagated down a probe, that serves as a transmission line, in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
With a TDR instrument using a single probe, it is necessary to couple the electronic circuitry to the transmission line so that the reflected pulses are separated from the transmitted pulses. One known type of circuit uses a resistance bridge such as is described in U.S. Pat. No. 5,517,198. The bridge couples a transmit pulse to a transmission line. The opposite side of the resistance bridge is coupled to a balancing load. Close-in performance can be enhanced by the use of this circuit in combination with a differential amplifier to cancel or null the transmit pulse from the detected output to allow improved close-in measurement. However, the transmit pulse amplitude is reduced by the resistance divider effect of the bridge. Also, some of the reflected signal bleeds over to the negative channel of the differential amplifier circuit which reduces receiver sensitivity. It is difficult to provide an excellent 50 ohm termination to the transmission line which may result in received pulses being re-reflected due to a less than perfect impedance match at the transmission line origin.
Other known products use variations of the resistance bridge and are adapted to peak or sharpen the transmit pulse. However, the reactance of such a circuit is a factor in impedance matching the transmission line termination. This makes a precise broadband match difficult to achieve.
Another known circuit uses an electronic microwave switch in the transmit/receive path. However the switch response is slow compared to signal propagation speeds. This type of circuit requires a long electrical delay line to give the switch time to operate before signals must be detected. Also, a microwave switch is a relatively expensive component.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.